1. Field of the Invention
The present invention relates to power amplifiers and, more particularly, to power amplifiers used in battery powered mobile handsets, and even more particularly to optimizing current use in such power amplifiers over a range of output power levels.
2. Background Information
Mobile handsets for wireless communication have become highly integrated to support new features such as multi-standard operation, camera, color display and high data rate transmission. The additional active components demand higher current consumption which, inevitably, shortens the battery life. On the other hand, wireless service providers and consumers demand long handset talk time from the next generation handsets. Hence, there is a constant need to reduce power consumption of the handset components in order to extend battery life.
The RF power amplifier is one of the main current consuming parts within handsets. Signals generated in a mobile handset are amplified by the power amplifier and sent to a base station for distribution to receivers. Often the frequency bands of operation of the handsets are predetermined, mainly in the frequency range from 800 MHz to 2000 MHz for various standards such as WCDMA (wide band code division multiple access) and CDMA (code division multiple access). The present invention, however, may find advantageous use at other frequencies and with other transmission standards.
In general, the handset is required to transmit at a high output power level when it is farther away from a receiving base station in order to maintain a pre-determined signal strength at the base station for sufficient reception. Conversely the closer the handset to the base station, less transmitted power would be required. Generally, the handset receives a control signal from the base station and, according to the embedded software command in this signal, adjusts the transmitted power accordingly. As known to those skilled in the art, the handset output signal has to meet the FCC regulation on spectral re-growth (as known as linearity—often measured in terms of adjacent channel leakage power ratio (ACLR) which stipulates the maximum allowable interference to other frequency channels in order to minimize interference between signals) with respect to the handset output power.
Handset power amplifiers are generally designed to meet the FCC spectral re-growth requirement at maximum transmit power level (+28 dBm for WCDMA system). Statistically, however, handsets transmit at maximum linear output power only for a small fraction of time, and most of the transmission takes place at a considerably lower power levels (10 dB or more below maximum linear power). The power added efficiency (PAE) of the conventional power amplifiers usually drops off rapidly under such backoff power operation, resulting in higher current draw coupled with excessive linearity margin. As known to those skilled in the art, the excessive linearity margin can be trade-off for low current consumption in power amplifiers.
U.S. Pat. No. 6,900,692 B2 ('692), to J. Kim et al., attacks this issue by disclosing a system that optimizes the output power levels at two discrete power output levels. Although, this improves overall efficiency, it remains inefficient at other power levels, and, as mobile devices proliferate, battery life is becoming more important in the competitive market.
U.S. Pat. No. 6,081,161 ('161) to F. L. Dacus et al. and U.S. Pat. No. 6,646,511 ('511) to J. C. Canyon et al. provide other solutions. Both of these patents teach monitoring the output power of the power amplifier through the current in the voltage bias line to the power amplifier. The detected current sets the optimized bias voltage for the power amplifier. However, the coupling loss (typically through a resistor or a current mirror) in the bias line would cause voltage drop especially under high current operation, reducing the bias voltage available to the power amplifier and resulting in performance degradation.
Another limitation of prior art power amplifiers ('511) is their use of external look up tables that are part of complex feedback loops for adjusting the power amplifier supply voltage. Such control schemes require additional calibration procedures and the operation is not adaptive within the power amplifier.
Another limitation of prior art power amplifiers ('161) is their dynamic control of the DC bias current through the gate node of the power amplifiers. As known to those skilled in the art, this technique is ineffective in reducing current consumption of the power amplifiers over different output power level as compared to reducing the collector or drain bias supply voltage of the power amplifier.
The prior art, generally, remains either inefficient in current consumption over a wide range of power or requires complex control circuitry with external input.
U.S. Pat. No. 6,900,697 ('697) to J. T. Doyle provides another solution. This patent teaches adjusting the supply voltage of the power amplifier based on an external power control signal and the output power of the power amplifier measured at the output of the power amplifier. The need of an external control signal makes the current consumption optimization non-adaptive which increases the control complexity.
The present invention is directed to dynamically optimizing current consumption and linearity of a power amplifier at each power level without the need for any external control in order to provide automatic current optimization for performance improvement while extending battery life.